Electrification is the most viable way to achieve clean and efficient transportation that is crucial to the sustainable development of the entire world. In the near future, electric vehicles (EV), including hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and pure battery electric vehicles (BEVs) will dominate the clean vehicle market. By 2020, it is expected that more than half of new vehicle sales will likely be EV models. The key and the enabling technology to this revolutionary change in transportation is the battery. EV batteries are quite different from those used in consumer electronic devices, such as laptops and cellphones. They are required to handle high power (up to a hundred kW) and have high energy capacity (up to tens of kW) within a limited space and weight and at an affordable price. The current two major battery types used in EVs today are nickel metal hydride (NiMH) and lithium secondary battery. Nearly all HEVs available in the market today use NiMH batteries because of its mature technology. Due to the potential of obtaining higher specific energy and energy density, the adoption of lithium secondary batteries is expected to grow fast in EVs, particularly in PHEVs and BEVs.
The negative electrode current collector of a lithium secondary battery is generally made using copper foil. The negative electrode is formed by coating the surface of the copper foil (current collector) with carbon powder, or other negative electrode active materials processed to a paste, drying this paste, then press flattening the negative electrode active materials by rolling or other pressure application. This composite of copper foil and compressed negative electrode active material is then, together with a separator, and an aluminum foil coated with a positive electrode active material, rolled up to form a cylindrical lithium secondary battery.
An alternative form of the lithium secondary battery is the laminated type lithium secondary battery. A laminated type lithium secondary battery boasts of advanced heat dissipation as compared to conventional cylindrical batteries. Because the laminated type lithium secondary battery has a broad surface area, the laminated type lithium secondary battery is better able to dissipate heat, and increases in the overall temperature of the battery due to charging and discharging can be kept low. Therefore, electric vehicles that adopt batteries of laminated type can simplify countermeasures against heat.
With an increase in the energy density and the capacity of the lithium secondary battery, and the need for the volumetric shrinkage rate of the negative electrode active material increasing, the required strength for the current collector is also increasing. Particularly, a metal alloy-based negative electrode active material containing silicon (“Si”), tin (“Sn”) or the like has several to several ten times the volumetric expansion higher than the conventional carbon material during the charging/discharging reaction.
For manufacturing a copper foil with stable, excellent properties for battery performance, one should impart to the copper foil during its manufacture various important properties. Included among such properties is a puncture strength that is suitable for employing the copper foil as a current collector in rechargeable secondary batteries of high capacity. With suitable puncture strength, the likelihood or probability that the copper foil will fracture during the charging/discharging cycle will be reduced. On the other hand, the negative electrode active material is closely adhered to the copper foil. Therefore, it suffers from the disadvantage that a large stress acts upon the negative electrode when the active material layer expands in volume due to the repetition of the charging/discharging cycle. When an electrode, with large expansion and contraction states is used in a rechargeable secondary battery, the charging/discharging cycle is repeated many times over the batteries' useful life repeatedly imparting stresses to the copper foil component of the electrode, whereby the negative electrode may be broken, resulting in a tremendous capacity reduction in the battery. For lithium secondary batteries with high capacity, which require a relatively high pressure to compress the negative electrode active materials onto the copper foil during production of the negative electrode, the copper foil is more likely to break during the pressing step of the manufacturing process of compressing the negative electrode active materials onto the copper foil.
After diligent research, the present inventors have found that copper foils suitable for use as current collectors in lithium secondary battery of high capacity must have a puncture strength within a range of values, but yet must not be too brittle as to fracture during the manufacturing of the consolidation of the negative electrode active material onto the copper foil by pressing. Various factors come into play when attempting to produce suitable copper foils for use in lithium secondary battery of high capacity as discussed in detail below. It is desirable to provide a rechargeable secondary battery, an electric tool, an electric vehicle, and a power storage system which can obtain an excellent battery capacity characteristic and cycle characteristics.